Segmented winding center-tap techniques for a coupled inductor circuit

ABSTRACT

Techniques are provided for a multiple-layer planar transformer having center taps of a segmented winding. In an example, a multiple-layer planar transformer can be a coupled inductor circuit including a first winding comprising a conductive coil having an electrical path defining and encircling the central axis, a second winding configured to magnetically couple with the first winding, the second winding having a plurality of individual segments, wherein each individual segment forms a fraction of one turn of the second winding, and a first output inductor coupled to a first common node of the second winding. The first common node can directly couple a first node of a first individual segment of the plurality of individual segments with a first node of a second individual segment of the plurality of individual segments.

TECHNICAL FIELD

This application provides techniques for coupled inductor circuits forDC-DC voltage converters or regulators.

BACKGROUND

DC-DC switching regulators, as the name applies, use high-frequencyswitching to generate a desired output voltage for an electronic device.In certain applications, the demand for low voltage electronics toaccept relatively high supply voltages creates design challenges forstepping down the supply voltage to a low supply voltage. The same, orvery similar, design challenges can also be found in step-upapplications where a high supply voltage is converted from a low inputsupply voltage.

SUMMARY OF THE DISCLOSURE

Techniques are provided for a multiple-layer planar transformer havingcenter taps of a segmented winding. In an example, a multiple-layerplanar transformer can be a coupled inductor circuit including a firstwinding comprising a conductive coil having an electrical path definingand encircling the central axis, a second winding configured tomagnetically couple with the first winding, the second winding having aplurality of individual segments, wherein each individual segment formsa fraction of one turn of the second winding, and a first outputinductor coupled to a first common node of the second winding. The firstcommon node can directly couple a first node of a first individualsegment of the plurality of individual segments with a first node of asecond individual segment of the plurality of individual segments.

This section is intended to provide an overview of subject matter of thepresent patent application. It is not intended to provide an exclusiveor exhaustive explanation of the invention. The detailed description isincluded to provide further information about the present patentapplication.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, which are not necessarily drawn to scale, like numeralsmay describe similar components in different views. Like numerals havingdifferent letter suffixes may represent different instances of similarcomponents. The drawings illustrate generally, by way of example, butnot by way of limitation, various embodiments discussed in the presentdocument.

FIG. 1 illustrates generally a schematic of an example planartransformer system according to the present subject matter.

FIG. 2 illustrates generally plots of signal waveforms illustrating theoperation of an example planar transformer system.

FIGS. 3A-3J illustrate generally example layouts for multiple layers ofa multiple-layer planar transformer system according the present subjectmatter.

FIGS. 4A-4C illustrate generally an example physical implementation of amultiple-layer planar transformer according to the present subjectmatter.

FIG. 5 illustrates generally a flowchart of an example method ofoperating a multiple-layer planar transformer with center taps of asegmented winding.

DETAILED DESCRIPTION

The present inventor(s) have recognized techniques for planartransformers, or planar coupled inductor circuits, that can employsegmented windings to reduce the size, and complexity of certainstep-down and step-up transformers for DC-to-DC voltage converterscompared to conventional techniques. In certain examples, planartransformers of the present subject matter can reduce the complexity ofnew designs such as the planar transformer designs of Paolucci, U.S.patent application Ser. No. 16/169,338, titled “SEGMENTED WINDINGTECHNIQUES FOR A COUPLED INDUCTOR CIRCUIT”, filed Oct. 24, 2018, whichis hereby incorporated by reference in its entirety. In addition, thepresent subject matter provides an architecture that allows for higherswitching frequencies that can result in reduced voltage stress acrossthe output inductors. The combination of higher switching frequency andlower voltage stress across the output inductors can provide asignificant reduction in ripple current through the output inductors andcan allow for use of air core output inductors which can providesignificant benefits compared to using metal core inductors. Suchbenefits can include, but are not limited to, lower cost, lower weight,smaller size, or combination thereof.

FIG. 1 illustrates generally a schematic of an example planartransformer system 100 according to the present subject matter. Theplanar transformer system 100 can include a planar transformer 101, afirst set of switches (Q1-Q4), a second set of switches (M5-M8), outputinductors 102. 103 and a controller 104. In some examples, each switchof the second set of switches can have an optional gate driver 110. Theplanar transformer 101 can include a core 105, a first winding 106, andone or more second windings 107, 108. The first winding 106 can includea number of turns and typically can include more than one turn. Eachsecond winding 107, 108 can include a number of winding segments 111,112, 113, 114. Each winding segment 111, 112, 113, 114 can form lessthan one turn of a second winding 107, 108. In certain examples, eachwinding segment can form significantly less than one turn of a secondwinding. In some examples, at least two or more winding segments arerequired to provide a single turn of one of the second windings. In someexamples, each winding segment can represent an integer fraction of asingle turn of the second winding, such as ½ of one turn, or ⅓ of oneturn, or ¼ of one turn, etc. As used herein, a winding segment includesthe primary conductive portion of a turn of a second winding and doesnot include ancillary parts of the second winding such as an externalterminal, a fuse, a switch, etc.

As used herein, the first winding can be either a primary winding of theplanar transformer or a secondary winding of the planar transformer. Forthis document, the first winding 106 is referenced as a primary windingunless noted otherwise. The first set of switches (Q1-Q4) are controlledby the controller and operate to periodically connect and disconnect theprimary winding 106 to voltage supply rails and to establish a cyclicalprimary voltage across the first winding 106 and a cyclical primarycurrent through the first winding 106. The second windings 107, 108 aremagnetically coupled to the first winding 106 via the core 105. The core105 can be an air core or a metal core.

The controller 104 can control the first set of switches (Q1-Q4) and thesecond set of switches (M5-M8) to provide an output voltage (V_(OUT))different than the input voltage (V_(IN)). In the illustrated planartransformer system 100, the output voltage (V_(OUT)) is lower than theinput voltage (V_(IN)) but the subject matter is not so limited. Whenthe planar transformer 101 is operated as a step-down transformer, thecontroller 104 can control the first set of switches (Q1-Q4) tooscillate a polarity of the input voltage (V_(IN)) across the firstwinding 106. When the planar transformer 101 is operated as a step-downtransformer, the controller 104 can synchronize the second set ofswitches (M5-M8) to extract power from the winding segments of thesecond winding. In the example system 100, the second det of switchescan be controlled with two phase signals (PH1, PH2). Details about howthe second set of switches (M5-M8) are controlled in such an example arediscussed below with respect to FIG. 3. In certain examples, each secondwinding 107, 108 can be configured to include one or more taps (E, F)between connected winding segments (111/112 and 113/114). In certainexamples, the tap (E, F) between winding segments can allow the outputvoltage (V_(OUT)) to include a step-up or step-down multiplier of theinput voltage (V_(IN)). In addition, compared to conventional methodsand even recent planar techniques, the tap (E, F) between the connectedwinding segments (111/112 and 113/114) can simplify the overall planartransformer design by using fewer switches. Furthermore, the tap (E, F)between the winding segments also allows for use of output inductors102, 103 with much lower inductance to smooth the output voltage(V_(OUT)). The lower inductance of the output inductors 102, 103 resultsfrom the circuit design using parasitic inductance of the other planartransformer components to smooth output ripple in the output voltage(V_(OUT)). The output inductors 102, 103 can be coupled between acorresponding tap (E, F) and an output voltage terminal of the planartransformer system 100. In certain examples, the output inductors 102,103 are air-core inductors.

FIG. 2 illustrates generally plots of signal waveforms illustrating theoperation of the planar transformer system of 100 of FIG. 1. The plotsinclude the logic level of the phase 1 (PH1) and phase 2 (PH2) signalsthat control the switches (M5-M8) of the second winding, the voltage(V₁) across the first winding, the current (I1) in the first winding,voltages (V_(A), V_(B), V_(C), V_(D)) at the extreme nodes (A,B,C,D) ofthe connected winding segments of the second winding, and the voltagesat the center taps (E, F) of the connected winding segments.

In general, the winding segments of the second winding are placed in oneof three phases to capture a voltage induced by the first winding duringthe transitions associated with the supply voltage being applied to, orisolated from the first winding. When the supply voltage is applied to,or isolated from, the first winding, the change in current through thefirst winding can induce a voltage across each second winding segment.By switching the connections of the second winding segments to capturethe voltage induced as current polarity of the first winding is changed,a stepped-down DC voltage can be captured at the center taps (E, F) ofthe connected winding segments, of the second winding. The plot ofsignals assumes that a logic high places each switch, or transistor, ina low impedance state (e.g., “on”) and a logic low places each switch ina high impedance state (e.g., “off”). However, it is understood thatswitches or transistors responding to logic commands differently arepossible and do not depart form the scope of the present subject matter.

For example, at to, the first winding circuit is in a first,free-wheeling state and the second winding circuit has all the switches(M5-M8) “on” (e.g., PH1=PH2=“high”), thus, coupling each extreme node(A, B, C, D) of the winding segments to ground. The free-wheeling stateof the first winding allows any current in the first winding to continueto flow until terminated by the circuit losses. Any current in theoutput inductors (e.g., FIG. 1, 102, 103) connected to the center tapnodes (E, F) is discharged to the output terminal, or charges the outputvoltage (V_(OUT)).

At t₁, the first winding circuit moves to the second state, and a supplyvoltage can be applied across the first winding with a first polarity(+V_(IN)). The application of the supply voltage (V_(IN)) can induce achange in current (I1) of the first winding and a voltage can be inducedacross the winding segments of the second windings. For example, at orin response to the application of the supply voltage (+V_(IN)) on thefirst winding, the switches (FIG. 1; M6, M8) associated with the phase 2control signal (PH2) can be turned “off”. The change in current (I1) offirst winding can induce a voltage at the drains (B, D) of the switches(FIG. 3; M6, M8) associated with the phase 2 control signal (PH2). Themagnetic coupling of the planar first and second windings can be quitegood such that the induced voltage (V_(B), V_(D)) of the segments of thesecond winding can match the sharp, pulse shape of the supply voltage(V_(IN)) applied to the first winding. The coupled winding segments forthis example each form one complete turn of each second winding. At t1,the phase 2 signal (PH2) is low and the associated switches (M6, M8) are“off”. The first winding 106, or primary winding for this step-downapplication, sees the full change in voltage across the windingterminals (e.g., 2*V_(IN)). Voltages across the connected windingsegments of each of the second windings is given by:

${V_{B} = {V_{D} = \frac{2 \cdot V_{IN}}{N \cdot S}}},$Where N is the turns ratio of the primary winding to each individualsecondary winding, and S is the number of winding segments in each turnof each secondary winding. As such, the voltages at the center tap nodescan be given by:

${V_{E} = {V_{F} = \frac{V_{IN}}{N \cdot S}}},$Assuming a load at the output terminal of the planar transformer system,current in the output inductors 102, 103 can increase due to positivevoltage across them.

At t₂, the controller can transition the first winding back to thefirst, free-wheeling state and the second winding circuit has all theswitches (M5-M8) “on” (e.g., PH1=PH2=“high”), thus, coupling eachextreme node of the winding segments of the second windings to ground.As before, any current flowing in the first winding continues to flowbecause the first winding inductance resists a change in current flow.The current may fall slightly during the free-wheeling state due tolosses in the circuit. Any current in the output inductors (e.g., FIG.1, 102, 103) connected to the center tap nodes (E, F) is discharged tothe output terminal, or charges the output voltage (V_(OUT)).

At t₃, the first winding circuit moves to the third state, and thesupply voltage (V_(IN)) can be applied across the first winding with asecond polarity (−V_(IN)). The application of the supply voltage(V_(IN)) can induce a change in current (I1) of the first winding andvoltage can be induced across segments of the second winding. Forexample, at or in response to the application of the supply voltage(−V_(IN)) on the first winding, the switches (M5, M7) associated withthe phase 1 control signal (PH1) can be turned “off”. The change incurrent (I1) of first winding can induce a voltage at the drains (A, C)of the switches (FIG. 3; M5, M7) associated with the phase 1 controlsignal (PH1). The magnetic coupling of the planar first and secondwindings can be quite good such that the induced voltage (V_(A), V_(C))of the segments of the second winding can match the sharp, pulse shapeof the supply voltage (V_(IN)) applied to the first winding. At t₃, thephase 1 signal (PH1) is low and the associated switches (M5, M7) are“off”. The first winding 106, or primary winding for this step-downapplication, sees the full change in voltage across the windingterminals (e.g., 2*V_(IN)). Voltages across the connected windingsegments of each of the second windings can be given by:

${V_{A} = {V_{C} = \frac{2 \cdot V_{IN}}{N \cdot S}}},$Where N is the turns ratio of the primary winding to each individualsecondary winding, and S is the number of winding segments in each turnof each secondary winding. As such, the voltages at the center tap nodescan be given by:

${V_{E} = {V_{F} = \frac{V_{IN}}{N \cdot S}}},$Assuming a load at the output terminal of the planar transformer system,current in the output inductors 102, 103 can increase due to positivevoltage across them.

FIGS. 3A-3J illustrate generally example layouts for multiple layers ofa multiple-layer planar transformer system according the present subjectmatter. In addition, the layouts include notations indicating nodescorresponding to the nodes shown in the example circuit of FIG. 1. INcertain examples, a winding segment can be formed using multiple windingsegments coupled in parallel. The planar transformer system example ofFIGS. 3A-3X use multiple winding segments coupled in parallel to form asingle winding segment as shown in FIG. 1 (e.g., 111, 112, 113, 114).The layouts also use multiple layers to form a single winding segment.The layouts of FIGS. 3A-3X show the conductive traces associated withthe planar transformer and area including vias for forming inter-layerconnections associated with those conductive traces. Ancillary tracerouting to the controller and the switches is not shown.

FIG. 3A illustrates generally a first layer 331 of a multiple-layerplanar transformer system. In certain examples, a major surface of thefirst layer 331 is an exposed as part of the overall system. The firstlayer includes areas to accommodate a core 306, a first portion 311A ofa first winding segment, a first portion 313A of a third windingsegment, a via area (A) for coupling directly with the first windingsegment, a via area (B) for coupling directly with a second windingsegment, a via area (C) for coupling directly with the third windingsegment, a via area (D) for coupling directly with a fourth windingsegment, a via area for a first center tap node (E), a via area for asecond center tap node (F), via areas 323, 324 for terminal connectionsto the first winding, and via areas 325, 326, 327 for intermediateconnections of the first winding. As the first area include an exposedsurface, FIG. 3A also shows the output inductors 302, 303. The firstoutput inductor 302 is coupled between the first center tap (E) and afirst output node 321. The second output inductor 303 is coupled betweenthe second center tap (F) and a second output node 322. In certainexamples, the first and second output nodes 321, 322 are electricallycoupled directly together to provide the output voltage (Vou-r).

FIG. 3B illustrates generally and second layer 332 of a multiple-layerplanar transformer system. The second layer includes first portion 306Aof the first winding. The first portion 306A provide conductive tracesbetween vias of a first area 323 of a first terminal node of the firstwinding and vias of a first intermediate node 325 of the first winding.In certain examples, vias of the first via area 323 can be selectivelycoupled to a supply rail of the multiple-layer planar transformersystem.

FIG. 3C illustrates generally a third layer 333 of the multiple-layerplanar transformer system. The third layer 333 includes areas toaccommodate a core 306, a first portion 312A of a second windingsegment, a first portion 314A of a fourth winding segment, a via area(A) for coupling directly with the first winding segment, a via area (B)for coupling directly with a second winding segment, a via area (C) forcoupling directly with the third winding segment, a via area (D) forcoupling directly with a fourth winding segment, a via area for a firstcenter tap node (E), a via area for a second center tap node (F), viaareas 323, 324 for terminal connections to the first winding, and viaareas 325, 326, 327 for intermediate connections of the first winding.

FIG. 3D illustrates generally a fourth layer 334 of a multiple-layerplanar transformer system. The fourth layer includes a second portion306B of the first winding. The second portion 306B provides a conductivetrace between vias of first intermediate node area 325 of the firstwinding and vias of a second intermediate node 326 of the first winding.

FIG. 3E illustrates generally a fifth layer 335 of a multiple-layerplanar transformer system. The fifth layer includes areas to accommodatea core 306, a second portion 311B of a first winding segment, a secondportion 313B of a third winding segment, a via area (A) for couplingdirectly with the first winding segment, a via area (B) for couplingdirectly with a second winding segment, a via area (C) for couplingdirectly with the third winding segment, a via area (D) for couplingdirectly with a fourth winding segment, a via area for a first centertap node (E), a via area for a second center tap node (F), via areas323, 324 for terminal connections to the first winding, and via areas325, 326, 327 for intermediate connections of the first winding.

FIG. 3F illustrates generally a sixth layer 336 of a multiple-layerplanar transformer system. The sixth layer includes a third portion 306Cof the first winding. The third portion 306C provides a conductive tracebetween vias of second intermediate node area 326 of the first windingand vias of a third intermediate node 327 of the first winding.

FIG. 3G illustrates generally a seventh layer 337 of the multiple-layerplanar transformer system. The seventh layer 337 includes areas toaccommodate a core 306, a second portion 312B of a second windingsegment, a second portion 314B of a fourth winding segment, a via area(A) for coupling directly with the first winding segment, a via area (B)for coupling directly with a second winding segment, a via area (C) forcoupling directly with the third winding segment, a via area (D) forcoupling directly with a fourth winding segment, a via area for a firstcenter tap node (E), a via area for a second center tap node (F), viaareas 323, 324 for terminal connections to the first winding, and viaareas 325, 326, 327 for intermediate connections of the first winding.

FIG. 3H illustrates generally an eighth layer 338 of a multiple-layerplanar transformer system. The eighth layer 338 includes areas toaccommodate a core 306, a third portion 311C of a first winding segment,a third portion 313C of a third winding segment, a via area (A) forcoupling directly with the first winding segment, a via area (B) forcoupling directly with a second winding segment, a via area (C) forcoupling directly with the third winding segment, a via area (D) forcoupling directly with a fourth winding segment, a via area for a firstcenter tap node (E), a via area for a second center tap node (F), viaareas 323, 324 for terminal connections to the first winding, and viaareas 325, 326, 327 for intermediate connections of the first winding.

FIG. 3I illustrates generally a ninth layer 339 of a multiple-layerplanar transformer system. The ninth layer includes a fourth portion306D of the first winding. The Fourth portion 306D provides a conductivetrace between vias of the third intermediate node area 327 of the firstwinding and vias of a second area 324 of a second terminal node of thefirst winding. In certain examples, vias of the via area 324 of thesecond terminal node can be selectively coupled to a supply rail of themultiple-layer planar transformer system.

FIG. 3J illustrates generally a tenth layer 340 of the multiple-layerplanar transformer system. The tenth layer 340 includes areas toaccommodate a core 306, a third portion 312C of a second windingsegment, a third portion 314C of a fourth winding segment, a via area(A) for coupling directly with the first winding segment, a via area (B)for coupling directly with a second winding segment, a via area (C) forcoupling directly with the third winding segment, a via area (D) forcoupling directly with a fourth winding segment, a via area for a firstcenter tap node (E), a via area for a second center tap node (F), viaareas 323, 324 for terminal connections to the first winding, and viaareas 325, 326, 327 for intermediate connections of the first winding.

FIGS. 4A-4C illustrate generally an example physical implementation 400of a multiple-layer planar transformer according to the present subjectmatter. FIG. 4A illustrates generally a top-view of the implementation400. Visible in the top view are the output inductors 402, 403, the topof the core 405, integrated circuits 451, 452 including drivers for thesecond set of switches, an integrated circuit 453 including switches forthe full bridge of the first winding first set of switches, an a digitalPWM controller 404. FIG. 4B illustrates generally a bottom view of theexample physical implementation 400. Visible in the bottom view are thebottom of the core 405, the switches (M5, M6, M7, M8) for the secondwinding, an integrated circuit 454 including an analog switch forcurrent sensing, and a multiple layer substrate 455. FIGS. 4A and 4Billustrate generally the locations of various terminals (A, B, C, D, E,F) of the physical implementation 400 of an example multiple-layerplanar transformer system. FIG. 4C illustrates a side view of theexample physical implementation 400. It is understood that the examplephysical implementation 400 illustrates only some of the components of amultiple-layer planar transformer and that the layout can includeadditional ancillary components such a resistor, capacitors, diodes,etc. without departing from the scope of the present subject matter.

FIG. 5 illustrates generally a flowchart of an example method 500 ofoperating a multiple-layer planar transformer. At 501, an input voltagewith a first polarity can be applied across a first winding of themultiple-layer planar transformer. At 503, in response to the voltageand polarity applied to the first winding, a first node of a firstsegment of a second, segmented winding can be electrically isolated fromground. In certain examples, a first transistor can couple the firstnode of the first segment to, and isolate the first node from, ground.At 505, a first node of a second segment of the second winding can beelectrically coupled to ground. In certain examples, a second transistorcan be used to couple the first node of the second segment to or isolateit from, ground. In certain examples, the first segment of the secondwinding forms only a portion of a conductive path of a single turn ofthe second winding, and the second segment of the second winding formsonly a portion of a conductive path of a single turn of the secondwinding. The first segment and the second segment can be coupled inseries at a first node common to both the first segment and to thesecond segment. At 507, varying magnetic field associated with the firstwinding can induce a voltage across the segments of the second windingand an output current can be extracted via a first output inductorhaving a first node coupled to a node common to the first segment and tothe second segment. In certain examples, each segment of the secondwinding can include conductive traces of several layers of themultiple-layer transformer coupled in parallel.

A second phase of the method can include applying the input voltageacross the first winding with opposite polarity, isolating the firstnode of the second segment from ground, coupling the first node of thefirst segment to ground, and extracting the output current via the firstoutput inductor. In certain examples, the multiple layer planartransformer can include an additional second winding having a firstsegment, second segment, first transistor, second transistor and asecond output inductor having a first node coupled to a node common toboth the first and second segment of the additional second winding. Thesecond node of the first output inductor and the second output inductorcan be coupled together to form an output terminal of the multiple-layerplanar transformer. Execution of the method of FIG. 5 with theadditional second winding and second output inductor can increase thecurrent supplied by the multiple-layer planar transformer.

A third phase of the method can occur during an idle period, orfreewheeling mode, between applications of the input voltage to thefirst winding. During the idle period, both nodes of the first windingcan both be coupled to one of the input voltage supply rails such thatthe nodes are coupled together. The current in the first winding canfreewheel with the nodes of the first winding coupled together. Also,during the idle period, the first nodes of each segment of each secondwinding can be coupled to ground. As such, output current of each outputinductor can be discharged to the output terminal of the multiple-layertransformer.

The above detailed description includes references to the accompanyingdrawings, which form a part of the detailed description. The drawingsshow, by way of illustration, specific embodiments in which theinvention can be practiced. These embodiments are also referred toherein as “examples.” Such examples can include elements in addition tothose shown or described. However, the present inventors alsocontemplate examples in which only those elements shown or described areprovided. Moreover, the present inventors also contemplate examplesusing any combination or permutation of those elements shown ordescribed (or one or more aspects thereof), either with respect to aparticular example (or one or more aspects thereof), or with respect toother examples (or one or more aspects thereof) shown or describedherein.

In the event of inconsistent usages between this document and anydocuments so incorporated by reference, the usage in this documentcontrols.

In this document, the terms “a” or “an” are used, as is common in patentdocuments, to include one or more than one, independent of any otherinstances or usages of “at least one” or “one or more.” In thisdocument, the term “or” is used to refer to a nonexclusive or, such that“A or B” includes “A but not B,” “B but not A,” and “A and B,” unlessotherwise indicated. In this document, the terms “including” and “inwhich” are used as the plain-English equivalents of the respective terms“comprising” and “wherein.” Also, the terms “including” and “comprising”are open-ended, that is, a system, device, article, composition,formulation, or process that includes elements in addition to thoselisted after such a term are still deemed to fall within the scope ofsubject matter discussed. Moreover, such as may appear in a claim, theterms “first,” “second,” and “third,” etc. are used merely as labels,and are not intended to impose numerical requirements on their objects.

Method examples described herein can be machine or computer-implementedat least in part. Some examples can include a computer-readable mediumor machine-readable medium encoded with instructions operable toconfigure an electronic device to perform methods as described in theabove examples. An implementation of such methods can include code, suchas microcode, assembly language code, a higher-level language code, orthe like. Such code can include computer readable instructions forperforming various methods. The code may form portions of computerprogram products. Further, in an example, the code can be tangiblystored on one or more volatile, non-transitory, or non-volatile tangiblecomputer-readable media, such as during execution or at other times.Examples of these tangible computer-readable media can include, but arenot limited to, hard disks, removable magnetic disks, removable opticaldisks (e.g., compact disks and digital video disks), magnetic cassettes,memory cards or sticks, random access memories (RAMs), read onlymemories (ROMs), and the like.

The above description is intended to be illustrative, and notrestrictive. For example, the above-described examples (or one or moreaspects thereof) may be used in combination with each other. Otherembodiments can be used, such as by one of ordinary skill in the artupon reviewing the above description. The Abstract is provided to complywith 37 C.F.R. § 1.72(b), to allow the reader to quickly ascertain thenature of the technical disclosure. It is submitted with theunderstanding that it will not be used to interpret or limit the scopeor meaning of a claim. Also, in the above Detailed Description, variousfeatures may be grouped together to streamline the disclosure. Thisshould not be interpreted as intending that an unclaimed disclosedfeature is essential to any claim. Rather, inventive subject matter maylie in less than all features of a particular disclosed embodiment. Thefollowing aspects are hereby incorporated into the Detailed Descriptionas examples or embodiments, with each aspect standing on its own as aseparate embodiment, and it is contemplated that such embodiments can becombined with each other in various combinations or permutations.

What is claimed is:
 1. A coupled inductor circuit comprising; a firstwinding comprising a conductive coil having an electrical path definingand encircling a central axis; a second winding configured tomagnetically couple with the first winding, the second winding having aplurality of individual segments, wherein each individual segment formsa fraction of one turn of the second winding; a first switch configuredto selectively couple a first supply rail of the coupled inductorcircuit with a first node of a first individual segment of the pluralityof individual segments; a second switch configured to selectively couplethe first supply rail of the coupled inductor circuit with a first nodeof a second individual segment of the plurality of individual segments;and a first output inductor coupled to a first common node of the secondwinding; wherein the first common node directly couples a second node ofthe first individual segment with a second node of the second individualsegment.
 2. The coupled inductor circuit of claim 1, wherein the firstindividual segment and the second individual segment define a firstconductive path about a majority of the central axis.
 3. The coupledinductor circuit of claim 2, wherein the first conductive path does notcompletely encircle the central axis.
 4. The coupled inductor circuit ofclaim 1, including a third winding configured to magnetically couplewith the first winding, the third winding having a second plurality ofindividual segments, wherein each individual segment of the secondplurality of individual segments forms a fraction of one turn of thethird winding.
 5. The coupled inductor circuit of claim 4, including asecond output inductor coupled to a first common node of the thirdwinding.
 6. The coupled inductor circuit of claim 5, wherein the centralaxis is located between the first common node of the second winding andthe first common node of the third winding.
 7. The coupled inductorcircuit of claim 5, wherein the first common node of the third windingdirectly couples a first node of a first individual segment of thesecond plurality of individual segments with a first node of a secondindividual segment of the second plurality of individual segments. 8.The coupled inductor circuit of claim 7, wherein the first individualsegment of the second plurality of individual segments and the secondindividual segment of the second plurality of individual segments definea second conductive path about a majority of the central axis.
 9. Thecoupled inductor circuit of claim 8, wherein the second conductive pathdoes not completely encircle the central axis.
 10. The coupled inductorcircuit of claim 7, including a first switch configured to selectivelycouple a second node of the first individual segment of the secondplurality of individual segments with a first supply rail of the coupledinductor circuit.
 11. The coupled inductor circuit of claim 10,including a second switch configured to selectively couple a second nodeof the second individual segment of the second plurality of individualsegments with the first supply rail of the coupled inductor circuit. 12.A method for operating a multiple-layer planar transformer, the methodcomprising: applying an input voltage having a first polarity across afirst winding to provide a first activation of the first winding, thefirst winding magnetically coupled to a second winding; isolating afirst node of a first segment of the second winding from a groundreference in response to the first activation; coupling a first node ofa second segment of the second winding to the ground reference;extracting output current via a first output inductor coupled to a firstnode common to the first segment and the second segment to provide anoutput voltage; wherein the first segment forms a first portion of afirst turn of the second winding; and wherein the second segment forms asecond portion of the first turn of the second winding.
 13. The methodof claim 12, including applying zero volts across the first winding inresponse to a conclusion of the first activation to provide a firstfreewheeling mode of the first winding.
 14. The method of claim 13,including coupling the first node of the first winding to zero inresponse to the first freewheeling mode; and discharging current of thefirst output inductor to provide the output voltage.
 15. The method ofclaim 14, including: applying the input voltage having a second polarityacross the first winding to provide a second activation of the firstwinding; isolating the first node of a second segment of the secondwinding from the ground reference in response to the second activation;coupling the first node of the first segment of the second winding tothe ground reference; and extracting output current via the first outputinductor coupled to the first node to provide the output voltage.
 16. Asystem comprising: a load; a multiple-layer planar transformerconfigured to provide power to the load, the multiple-layer planartransformer including: a first winding comprising a conductive coilhaving an electrical path defining and encircling a central axis; asecond winding configured to magnetically couple with the first winding,the second winding having a plurality of individual segments, whereineach individual segment forms a fraction of one turn of the secondwinding; a first output inductor coupled to a first common node of twosegments of the second winding; a first switch configured to selectivelycouple a first supply rail with a first node of a first individualsegment of the plurality of individual segments; and a second switchconfigured to selectively couple the first supply rail with a first nodeof a second individual segment of the plurality of individual segments.17. The system of claim 16, wherein the first output inductor is anair-core inductor.
 18. The system of claim 16, including: an additionalsecond winding configured to magnetically couple with the first winding,the additional second winding having a second plurality of individualsegments, wherein each individual segment of the second plurality ofindividual segments forms a fraction of one turn of the additionalsecond winding; a second output inductor coupled to a first common nodeof two segments of the additional second winding; and wherein a node ofthe first output inductor is coupled to a node of the second outputinductor to provide an output node to provide the power to the load.